This invention relates to lathes and more particularly to lathes that sharpen the cylindrical grinding stones used in pulp-making machinery.
Since the invention of "pulp" paper, wood has been traditionally ground into pulp. This soft, moist mass is then combined with various additional fibers to produce the type of paper most often used today.
To make the pulp, large grinders have been used. One such machine is the Great Northern Waterous Grinder manufactured by the assignee of the present invention, Montague Industries, Inc. (d.b.a. Montague Machine Co.) of Turners Falls, Mass.
To use the "Great Northern" (shown in FIG. 1), logs are first loaded into a top chute or hopper. From there, they fall into an underlying pocket or grinding chamber where they are pressed against a cylindrical grinding stone by a hydraulically operated piston. The stone then rotates to break down the logs and grind them into fine particles. As it does, the particles are mixed with water to form the pulp.
The "pulp stone" has a series of helical grooves in its grinding surface. These grooves spiral around the stone's central, rotational axis and, though they intersect, they are parallel to one another.
During the stone's rotation, these grooves help to break down the logs. In addition to initially catching the log ends to assist in snapping the logs, they also provide a series of sharp, helical cutting edges or shoulders to subsequently slice large log pieces into small ones.
Over time, the outer surface of the cylindrical stone becomes worn, the side edges of the grooves become dull and the grooves become shallow. At that point, the helical pattern on the stone needs to be resharpened or "dressed".
As with most grinders, the Great Northern Waterous Grinder comes equipped with its own hydraulically operated lathe. This lathe has a dressing wheel which is mounted on a hydraulically movable carriage that travels axially across the top of the grinding stone. As it moves, the wheel hopefully recuts the helical pattern that has been eroded. Ideally, the shoulders of each groove should be squared and the grooves made deep again.
The cutting surface on the wheel comprises several parallel helices of discrete burr teeth. Each set of teeth or burrs is supposed to ride in an underlying aligned groove during an axial movement of the wheel across the stone.
It is important that these burrs constantly mesh with the old grooves during resharpening. If the linear velocity of the carriage-carrying wheel changes during a pass across the stone, the helices of the burrs and grooves will not align and the wheel tends to pop out of the grooves.
One of the problems with the water-driven system is that the water pressure fluctuates. This causes a velocity change and results in the burrs leaving the grooves and scarring the stone.
Another problem is caused by creepage of the carrier when the water source is turned off. Due to internal build-up of pressure inside the system's hydraulic cylinder, the system's carriage-moving piston tends to edge forward when the water is shut off. This can cause the burrs to slowly leave the grooves and chip the grooves' lead shoulders.
Maintenance problems also occur with all the pulp grinders that use the prior hydraulic system. Since the speed of each lathe varies as the particular system's packings and cylinder lining wear, the system needs to be constantly monitored for wearage. Otherwise, "last months" water pressure may cause the carriage to move too quickly.